NSW Premier Gladys Berejiklian has apologised to the seven-year-old girl allegedly assaulted by a sex offender on parole, as the NSW Police Commissioner, Mick Fuller, admitted the organisation had likely "failed" to act on complaints weeks earlier that the man had breached the conditions of his release.

Ms Berejiklian said it was "absolutely not acceptable" for police standards to "slip" as she apologised to "everybody affected".

"On this occasion, madam Speaker, getting it wrong may have resulted in the most terrible of circumstances and we are deeply, deeply sorry to everybody affected," Ms Berejiklian said in Parliament on Thursday.

"I cannot imagine the pain and suffering of the victims involved and the impact on their families and loved ones."

"It will be alleged [Anthony] Sampieri has breached parole and potentially reoffended under the Telecommunications Act," Commissioner Fuller said in a press conference on Thursday.

"A victim attended St George police station on 26th October and a police officer had taken a complaint.

"We are currently investigating the response to those victim's concerns but at this stage certainly, at best, the organisation potentially has failed to notify the parole board of a potential parole breach and certainly, at worst, an officer has neglected their duty to a victim of NSW."

Police Minister Troy Grant said he was "very very disappointed" and "sick in the guts" at the circumstances which had come to light so far but said the investigation needed to run its due course.

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"I think the whole community feels that way and I'm not trying to undermine any investigations – all that has to happen appropriately and within the rules – but the nature of this conversation makes me crook in the guts," Mr Grant said.

Opposition Leader Michael Daley said he would launch a "full, open and public inquiry" into the NSW parole system as his first act as Premier if Labor won government at the state election in March.

Mr Sampieri was on parole in connection with a seven-year sentence for sexually assaulting a 60-year-old woman at knifepoint in her Illawarra unit in 2012, when he allegedly sexually assaulted a seven-year-old girl in Kogarah last week.

The 54-year-old allegedly sexually and physically assaulted the girl in the toilets of a Kogarah dance studio before two other men confronted the alleged offender and detained him.

One of the men who came to her aid, a father of another dance school student, was allegedly stabbed several times during the struggle, including in the stomach and neck.

Just weeks earlier, on October 26, a woman reported offensive phone calls made to her at a business in Sydney's south by Mr Sampieri.

Commissioner Fuller said the October complaint was "something that we should have made contact with our local parole officers [about] and notified them of our concerns and then they would have conducted an investigation to see whether there was a breach of parole or not."

He said he was first notified on Tuesday afternoon that Mr Sampieri had potentially breached parole before the alleged assault and "immediately" launched an investigation into the police response.

He said the officer at the centre of the police investigation was "stressed" and "concerned".

"We still have an obligation for that officer's welfare, we won't walk away from him, but if there needs to be accountability, the community would expect there is accountability for this," Commissioner Fuller said.

The officer has been placed on non-operational duties and will have no professional contact with members of the public.

Commissioner Fuller said Mr Sampieri remained under police guard in hospital and there was "a real chance that there will be fresh charges waiting for him on the basis of the investigation into potential other criminal activities".